A phenomenon in which a solvent moves through a separator membrane to pass between two solutions separated by a semipermeable membrane, from a solution in which a concentration of a solute is low to a solution in which a concentration of a solute is high, is known as osmosis, and here, pressure acting on the solution in which the concentration of a solute is high due to the movement of the solvent is known as osmotic pressure. In this case, when external pressure, higher than osmotic pressure, is applied, the solvent moves toward a solution in which the concentration of a solute is low, and this phenomenon is known as reverse osmosis. Various salts or organic materials may be separated from a solution by the semipermeable membrane by using a pressure gradient as driving force on the basis of the reverse osmosis principle. A reverse osmosis membrane using a reverse osmosis phenomenon has been used to separate molecular-level material and remove salt from brackish water or sea water and supply water for domestic, commercial and industrial use.
A typical example of a reverse osmosis membrane is a polyamide-based reverse osmosis membrane. The polyamide-based reverse osmosis membrane is fabricated by a method of forming a polyamide active layer on a microporous layer support. In detail, a microporous support is formed by forming a polysulfone layer on a non-woven fabric, dipping the microporous support in an aqueous m-phenylene diamine (mDP) solution to form an mPD layer, dipping the mPD layer in an organic trimesoyl chloride (TMC) solvent to allow the mPD layer to be brought into contact with the TMC so as to be interfacially polymerized to form a polyamide layer.
Meanwhile, recently, in order to improve the performance of a reverse osmosis membrane, various raw water modification and membrane postprocessing techniques such as a method of undertaking graft polymerization on a surface of a reverse osmosis membrane by using an atmospheric plasma and vinyl monomer to enhance durability or anti-contamination properties of a polyamide layer (US 2009/0308804 A1), a method of preliminarily minimizing fouling by adding acidic polysaccharide to a feeding liquid (US 2009/0188861 A1), a method of introducing inorganic particles such as TiO2, or the like, to an amide film (U.S. Pat. No. 6,551,536) have been developed.
However, these techniques show an insignificant addition effect or, since several coating layers are formed, a thickness of a reverse osmosis membrane is increased to degrade water purifying performance. In addition, a process of adding various additives to raw water, or the like, complicates a manufacturing process.
Also, as related art, a method of providing functionality by performing a surface treatment process with a surface treatment solution including silver nanoparticles dispersed therein on a reverse osmosis membrane module after manufacturing the reverse osmosis membrane module (Korean Patent Laid Open Publication No. 2010-0079530) has been proposed. However, when the reverse osmosis membrane module manufactured through the foregoing method is used in actuality on the spot, since the silver nanoparticles are mostly exposed from a surface of the reverse osmosis membrane, the silver nanoparticles are highly likely to be eluted under operational conditions of high pressure and high speed. In consideration of the revision limiting the use of two types of nano materials (silver nano and carbon nanotubes (CNT)) among existing RoHS regulated substances in the EU, taken at an environmental committee conference held on Jun. 2, 2010, the practical process of introducing silver nanoparticles to a reverse osmosis membrane through a surface treatment method is anticipated to have great difficulty in the application thereof, due to the risk of the elution of silver nanoparticles, in spite of the excellent and unique properties of silver in such as antibiotic applications, anti-contamination applications, chlorine-resistance applications, and the like.
Also, as related art, a method of directly adding silver nanoparticles in forming an ultrafilter (UF) membrane, a microfilter (MF) membrane, or a separator membrane has been attempted. However, silver nanoparticles have a particle shape (a spherical shape or an angular shape) having a surface area smaller than that of a silver nanowire. Thus, for example, in the case of a reverse osmosis membrane, silver nanoparticles are not exposed nor expressed to an outer side of a polyamide film, having difficulty in obtaining an intrinsic effect of silver.